Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

Metal nanoparticles stabilized on a support material catalyse many major industrial reactions. Metal-support interactions in these nanomaterials can have a substantial influence on the catalysis, making metal-support interaction modulation one of the few tools able to enhance catalytic performance. This topic has received much attention in recent years, however, a systematic rationalization of the field is lacking due to the great diversity in catalysts, reactions and modification strategies. In this review, we cover and categorize the recent progress in metal-support interaction tuning strategies to enhance catalytic performance for various reactions. Furthermore, we quantify the productivity enhancements resulting from metal-support interaction control that have been achieved in C1 chemistry in recent years. Our analysis shows that up to fifteen-fold productivity enhancement has been achieved, and that metal-support interaction is most impactful for metal nanoparticles smaller than four nanometres. These findings demonstrate the importance of metal-support interaction to improve performance in catalysis. Methods to control the performance of heterogeneous catalysts are extremely relevant to the success of industrial processes. This review provides a rationalization of the effects that metal support interactions have on the reactivity of different catalytic systems, emphasizing strategies to tune such effects.

Control of metal-support interactions in heterogeneous catalysts to enhance activity and selectivity

Oxygen defects and surface chemistry of ceria: quantum chemical studies compared to experiment.

Probing surface and interface morphology with Grazing Incidence Small Angle X-Ray Scattering

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Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

The ability of Ce-containing bioactive glasses to inhibit oxidative stress in terms of reduction of hydrogen peroxide, by mimicking the catalase enzyme activity is demonstrated here for the first time. The antioxidant properties of three bioactive glasses containing an increasing amount of CeO2 have been evaluated by following the degradation of hydrogen peroxide with time after immersion in H2O2 aqueous solutions with different concentration. XPS and UV–vis measurements allowed us to determine the Ce3+/Ce4+ ratio in the bulk and on the glass surface, and to correlate it with the ability of the samples to show catalase mimetic activity. Interestingly, we have found that the bioactive glass with composition 23.2Na2O–25.7CaO–43.4SiO2–2.4P2O5–5.3CeO2 immersed in 0.1 M H2O2 aqueous solution is able to degrade 90% of it in 1 week. The reduction in bioactivity of the glasses with increasing CeO2 content is here rationalized in terms of a lower amount of phosphate groups available for the hydroxyapatite layer fo...

Evidence of Catalase Mimetic Activity in Ce3+/Ce4+ Doped Bioactive Glasses

Ag nanoparticles have been deposited on stoichiometric and reduced thin CeO2 films grown on Pt(111). The nucleation and growth of the Ag nanoparticles has been characterized by STM and XPS (X-ray photoemission spectroscopy) measurements complemented with DFT calculations on Ag atoms, clusters, and extended layers deposited on slab models of the CeO2(111) surface. The XPS spectra clearly show a reduction of the ceria support by Ag deposition (formation of Ce3+ ions). This is accompanied by a positive shift of the Ag 3d core levels, in which final state effects related to the finite size of the Ag deposits come into play. The DFT calculations support the view of a direct electron transfer from the Ag clusters and nanoparticles to the ceria support. Other possible origins of the reduction of the ceria substrate, like the occurrence of oxygen reverse spillover on the Ag nanoparticles, are ruled out based on energy considerations.

Nature of Ag Islands and Nanoparticles on the CeO2(111) Surface

The morphology of ultrathin epitaxial MgO layers reactively grown on Ag(001) was investigated by using scanning tunnelling microscopy. In the initial deposition stage Ag atoms are partially removed from the substrate and form extended monoatomic islands leaving vacancy islands in the substrate itself. On individual substrate terraces Ag is thereafter found at three atomic levels. For submonolayer deposition MgO condensates in form of small islands of monoatomic height, located on the original substrate, on the protruding Ag islands and on the vacancy islands as well. The largest Ag(001) fractional coverage by monoatomic MgO islands is 70%. A limited amount of MgO bilayers or trilayers has also been detected (about 1% fractional coverage). At the nominal deposition of 1 ML flat, squared and connected MgO domains of about 10 nm in width form, with edges along the [110] directions. The actual substrate fractional coverage is about 85% and the occurrence of bilayers and multilayers becomes significant (about 30 and 5% fractional coverage, respectively), resulting in the formation of three-dimensional pyramidal MgO islands.

Scanning tunnelling microscopy of MgO ultrathin films on Ag(001)

Geometric parameters of NiO films epitaxially grown on Ag(001) were determined using two independent experimental techniques and ab initio simulations. Primary beam diffraction modulated electron emission experiments determined that the NiO films grow with O on top of Ag and that the oxide/metal interface distance is d=2.3+/-0.1 A. Polarization-dependent x-ray absorption, at the Ni-K edge, determined the tetragonal strain (r( parallel )=2.046+/-0.009 A, r( perpendicular )=2.12+/-0.02 A) and d=2.37+/-0.05 A. Periodic slab model results agree with the experiments (d=2.40, r( parallel )=2.07, r( perpendicular )=2.10 A; the O-on-top configuration is the most stable).

Oxide/metal interface distance and epitaxial strain in the NiO/Ag(001) system.

Studies of model systems based on cerium oxide are important to improve current understanding of the properties of ceria-based materials, which find wide application based on the ability of cerium oxide to store, release, and transport oxygen. We report a study of CeO2 ultrathin films grown on the Pt(111) surface by reactive deposition of Ce using molecular or atomic oxygen as the oxidizing gas. High-temperature treatments in O2 allowed us to obtain epitaxial structures with a very good quality in terms of morphology, stoichiometry, and structure. The cerium oxide films have a very flat morphology with terraces several tens of nanometers wide. The stoichiometry of the films is mainly CeO2, and the concentration of Ce3+ ions in the film can be reversibly increased by temperature treatments. We propose that the Pt substrate oxidation has a determinant role for the epitaxial stabilization of ceria films.

Paola Luches

Papers

Evidence of Catalase Mimetic Activity in Ce3+/Ce4+ Doped Bioactive Glasses

Nature of Ag Islands and Nanoparticles on the CeO2(111) Surface

Scanning tunnelling microscopy of MgO ultrathin films on Ag(001)

Oxide/metal interface distance and epitaxial strain in the NiO/Ag(001) system.

Morphology, Stoichiometry, and Interface Structure of CeO2 Ultrathin Films on Pt(111)